A liquid modified photonic crystal fiber (PCF) integrated with an embedded directional coupler and multi-mode interferometer is fabricated by infiltrating three adjacent air holes of the innermost layer with standard 1.48 refractive index liquids. The refractive index of the filled liquid is higher than that of background silica, which can not only support the transmitting rod modes but also the “liquid modified core” modes propagating between the PCF core and the liquid rods. Hence, the light propagating in the liquid modified core can be efficiently coupled into the satellite waveguides under the phase-matching conditions, resulting in a dramatic decrease of the resonant wavelength intensity. Furthermore, there is a multi-mode interference produced by modified core modes and rod modes. Such a compact ($\sim 0.91\mathrm{cm}$) device integrated with an embedded coupler and interferometer is demonstrated for high-sensitivity simultaneous temperature ($\sim 14.72\mathrm{nm}/°\mathrm{C}$) and strain ($\sim 13.01\mathrm{pm}/\mathrm{\mu ϵ}$) measurement.

Ultra-violet (UV) light has still a limited scope in optical microscopy despite its potential advantages over visible light in terms of optical resolution and of interaction with a wide variety of biological molecules. The main challenge is to control in a robust, compact and cost-effective way UV light beams at the level of a single optical spatial mode and concomitantly to minimize the light propagation loss. To tackle this challenge, we present here photonic integrated circuits made of aluminum oxide thin layers that are compatible with both UV light and high-volume manufacturing. These photonic circuits designed at a wavelength of 360 nm enable super-resolved structured illumination microscopy with conventional wide-field microscopes and without modifying the usual protocol for handling the object to be imaged. As a biological application, we show that our UV photonic chips enable to image the autofluorescence of yeast cells and reveal features unresolved with standard wide-field microscopy.

A new fiber interface Mach–Zehnder interferometer has been fabricated, to the best of our knowledge, in coreless fiber by femtosecond laser-inscription for temperature-insensitive refractive index measurement. A straight waveguide was inscribed along the central axis of the coreless fiber as the reference arm, and the other curved waveguide (interface waveguide) was then inscribed bending toward the cladding interface to obtain a strong evanescent field sensitive to ambient refractive index. This fiber interface Mach–Zehnder interferometer exhibits a high refractive index (RI) sensitivity of $\sim 3000\mathrm{nm}/\mathrm{RIU}$ at an RI value of 1.432. Moreover, with the significant advantages of high mechanical strength and temperature independence, such a fiber Mach–Zehnder interferometer may find many potential applications in biochemical sensing.

A fiber surface Bragg grating waveguide (BGW) fabricated in the surface of single-mode fiber by direct femtosecond laser inscription is demonstrated and successfully applied for refractive index (RI) measurements. Prior to laser inscription of the fiber surface BGW, an $X$-coupler is first inscribed across the fiber core to couple light from the core to the fiber surface. The light transmitted in the fiber surface BGW efficiently interacts with the surrounding medium due to a strong evanescent field, and obtains an acceptable RI sensitivity approaching $\sim 16\mathrm{nm}/\mathrm{RIU}$. The novel design efficiently couples the light guided in the core with the surrounding medium using a non-destructive, single-step micromachining process, and is expected to have potential applications in fiber biochemical sensing.

A novel surface plasmon resonance (SPR) configuration based on fiber-interface waveguide was proposed and realized by combining the technology of femtosecond laser writing waveguide with SPR effect for measuring refractive index (RI) of analyte. A U-shaped waveguide is inscribed in the coreless fiber and its bottom is very close to the fiber surface, which can produce strong evanescent field being sensitive to ambient media. When the fiber surface is coated with a layer of gold film, the strong evanescent field can excite the SPR effect on the fiber surface. Most importantly, different from some types of fiber SPR sensors with a fragile physical structure, the fiber-interface waveguide SPR sensor exhibits an excellent mechanical strength. Such a SPR sensor exhibits a high sensitivity of $\sim 3352\mathrm{nm}/\mathrm{RIU}$ at the RI value of $\sim 1.395$, which may have important practical applications in medicine, environmental monitoring, and food safety.

Discriminating edible oils from gutter oils has significance in food safety, as illegal gutter oils cannot meet a variety of criteria such as the acid value, peroxide value and quality. To discriminate these illegal cooking oils, we propose an ultrasensitive optofluidic detection method based on a hybrid-waveguide coupler. Prior to the straight waveguide inscription in the cladding of the silica tube using a femtosecond laser, a section of coreless fibre is firstly spliced with the ST to supply a platform for the inscription of an S-band waveguide. Then a pair of microfluidic channels are ablated on the ST using the fs laser to enable liquid analytes to flow in and out of the air channel. In the transmission spectrum, a unique resonant loss dip can be observed, which is produced by coupling the light from the laser inscribed waveguide to the liquid core when the phase-matching condition is met. This hybrid-waveguide coupler with a simplified structure realizes dynamic optofluidic refractive index sensing with an ultrahigh sensitivity of −112 743 nm RIU⁻¹, a detection limit of 2.08 × 10⁻⁵ RIU and a refractive index detection range from 1.4591 to 1.4622. This novel method can be used for food safety detection, specifically, for the discrimination of gutter oils.